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BUREAU OF MINES 
INFORMATION CIRCULAR/1989 



Safety Evaluations of Longwall 
Roof Supports 



By Thomas M. Barczak 



UNITED STATES DEPARTMENT OF THE INTERIOR 



Mission: As the Nation's principal conservation 
agency, the Department of the Interior has respon- 
sibility for most of our nationally-owned public 
lands and natural and cultural resources. This 
includes fostering wise use of our land and water 
resources, protecting our fish and wildlife, pre- 
serving the environmental and cultural values of 
our national parks and historical places, and pro- 
viding for the enjoyment of life through outdoor 
recreation. The Department assesses our energy 
and mineral resources and works to assure that 
their development is in the best interests of all 
our people. The Department also promotes the 
goals of the Take Pride in America campaign by 
encouraging stewardship and citizen responsibil- 
ityforthe public landsand promoting citizen par- 
ticipation in their care. The Department also has 
a major responsibility for American Indian reser- 
vation communities and for people who live in 
Island Territories under U.S. Administration. 



C^Ju/M^, &*ruw /fa**; 



Information Circular 9221 




Safety Evaluations of Longwall 
Roof Supports 



By Thomas M. Barczak 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Manuel J. Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 



1\\ 






^ 







5 



Library of Congress Cataloging in Publication Data: 




Barczak, Thomas M. 

Safety evaluations of longwall roof supports. 

(Bureau of Mines information circular, 9221) 

Bibliography: p. 17 

Supt. of Docs, no.: I 28.27:9221. 

1. Mine roof control-Evaluation. 2. Longwall mining. I. Title. II. Series: 
Information circular (United States. Bureau of Mines); 9221. 

TN295.U4 [TN288] 622 s [622\334] 89-600003 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Support design and mechanics 4 

Failure mechanisms 8 

Inadequate support capacity 9 

Structural failure 9 

Base failures 10 

Canopy failures 10 

Failures of caving shield and links 11 

Leg cylinder failures 11 

Instability 11 

Safety precautions 12 

Support capacity considerations 12 

Structural failure and stability considerations 13 

Hypothetical failures 13 

Case studies of support failures 14 

Future research efforts 17 

Conclusions 17 

References 17 

ILLUSTRATIONS 

1. Bureau's mine roof simulator 3 

2. Two-dimensional diagram of shield support 4 

3. Three-dimensional representation of longwall shield 4 

4. Shield component construction 5 

5. Vertical and horizontal loads acting on shield support 6 

6. Gob loading on caving shield 7 

7. Load conditions (displacements) imposed on shield support 8 

8. Lateral loading caused by uneven roof conditions along face 8 

9. Conditions that produce maximum loading in support components 8 

10. Shield supports going solid from inadequate capacity 9 

11. Relationship between strata convergence and support setting force 12 

12. Failure of leg cylinder casing 14 

13. Split caving shield design that failed from instability 14 

14. Permanent deformation of base structure 14 

15. Example of leg socket failure 15 

16. Base failure and subsequent modification 16 

17. Failure of canopy structure from high stresses in canopy hinge 16 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


ft 


foot pet percent 


in 


inch psi pound per square inch 


ksi 


kip per square inch 



SAFETY EVALUATIONS OF LONGWALL ROOF SUPPORTS 

By Thomas M. Barczak 1 



ABSTRACT 

State-of-the-art longwall roof supports provide effective strata control, but failures of these support 
systems still occur. To identify failure mechanisms and the impact these failures have on the safety of 
the support system, the U.S. Bureau of Mines has been conducting research on shield mechanics in the 
Bureau's unique mine roof simulator, as well as field studies of in situ support loading and investigations 
of longwall failures. Three types of failures are discussed: (1) inadequate support capacity, 
(2) structural failure, and (3) instability. Hypothetical situations with proposed courses of action and 
case studies of actual longwall failures are described. This information is intended to assist the Mine 
Safety and Health Administration (MSHA) and industry mining personnel in safety evaluations of 
longwall roof support systems. 



Research physicist, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



Longwall roof supports are robust structures that are 
designed to provide effective ground control during the 
extraction of a number of longwall panels. A typical life 
expectancy for these support systems is 7 to 10 longwall 
panels, which constitute 14,000 to 20,000 operating cycles, 
assuming a 30-in cut per shearer pass in panels 5,000 ft in 
length. Failure of the support system can be attributed to 
one of three things: (1) inadequate support capacity, where 
support resistance is insufficient to prevent strata con- 
vergence beyond the hydraulic yield of the leg cylinders; 
(2) structural failure caused by either fatigue or compo- 
nent loading beyond its design strength; or (3) instability 
caused by structural failure of support components, exces- 
sive wear in the pin joints of the structure, or load condi- 
tions incompatible with the support design. 

Many of these failures develop gradually and usually do 
not present an immediate safety hazard in their early 
stages of development. However, as the failure progresses 
to the point where the structural integrity of the support is 
compromised, the capability of the support to provide 
effective ground control and safety to the miners is 
endangered. Such circumstances could result in cata- 
strophic failure, with potential loss of life. Further, many 
incipient failures are difficult to detect while the support is 
in service underground. Typically, supports are not rou- 
tinely inspected for failure, and problems are most often 
detected during advanced stages, long after failure has 
begun. 

As part of its program to improve mining safety, the 
Bureau of Mines has been conducting research on shield 
mechanics for the past 5 years, in the Bureau's unique 
mine roof simulator shown in figure 1. These 



nondestructive tests provide a basic understanding of sup- 
port responses and load transfer mechanics for various 
load conditions. In addition, the Bureau has conducted 
several tests in cooperation with participating coal 
operators and support manufacturers to duplicate observed 
in-service support failures in the simulator. Field 
observations of support failures by the Bureau have also 
provided experiences that contribute to these discussions 
of safety evaluations of longwall roof supports. The objec- 
tives of this information circular are to identify the signs of 
failure, describe the mechanisms that cause support failure, 
and provide some recommendations for courses of action 
pertaining to continued operation of support systems that 
are in the process of failure. 

Longwall mining in the United States is now at the 
point where many of the existing face support systems are 
approaching the end of their service lives. Many mining 
companies are trying to extend the life of their equipment 
because of financial hardships associated with current 
difficult market conditions in the coal mining industry. 
These developments make it likely that the number of 
longwall support equipment failures will grow in the next 
3 to 5 years. Mine operators and MSHA inspectors must 
make difficult judgments as to the safety of longwall faces. 
These people do an excellent job of protecting the 
American miners, and it is hoped that this information 
circular will assist them in this goal by pointing out what 
to look for and what to expect for the types of support 
failures likely to be encountered underground. It should 
also make mine personnel more aware of the potential 
hazards associated with longwall roof support and assist 
them in the initial selection of equipment. 




Figure 1. -Bureau's mine roof simulator. 



SUPPORT DESIGN AND MECHANICS 



Before support failures are discussed, a brief summary 
of support design requirements and support mechanics is 
presented. A more detailed presentation of this material 
can be found in various other Bureau publications {1-4). } 
The objectives here are to provide only a basic under- 
standing of why the support is designed as it is, how it 



Italic numbers in parentheses refer to items in the list of references 



at the end of this report. 
Canopy 




Leg cylind 



\ \ capsule ' / •) 

link 




Base 
Figure 2. -Two-dimensional diagram of shield support. 



transfers loads imposed by the strata, and what conditions 
cause maximum loading of the support components. This 
discussion will be limited to shield designs with lemniscate 
linkage in the caving shield. All shields purchased within 
the past 10 years, both two-leg and four-leg designs, utilize 
the lemniscate link design to maintain a constant tip-to- 
face span throughout the operating range of the support. 
Approximately 70 pet of existing longwall faces employ 
two-leg shield supports. 

A two-dimensional diagram identifying the components 
of a two-legged longwall shield is shown in figure 2. The 
canopy and caving shield span the width of the support in 
the third dimension, and there are both left-side and right- 
side leg cylinders, front links, and rear links, as depicted in 
figure 3. Most bases are also of a split configuration, 
incorporating a left side and a right side. 

Canopies and bases are constructed as stiffened plates, 
with a top and bottom plate separated by internal stiff- 
eners. The plate structure provides bending strength while 
the stiffeners transfer shear. Bending is the primary 
loading mechanism for canopy and base components. Links 
are primarily designed for axial loading and are generally 
constructed as open or solid box sections. The caving 
shield is also a stiffened plate design, with bending as the 




Figure 3.-Three-dimensional representation of longwall shield. 



primary loading mechanism. These component construc- 
tions are illustrated in figure 4. Reference to these com- 
ponent constructions will be made when discussing support 
failures. 

Shield supports are designed to provide resistance to 
both vertical and horizontal loading imposed by the strata, 
as illustrated in figure 5. In addition, shield supports 
employ a caving shield to prevent gob debris from entering 
the face area; hence, the structure must be designed for 
loads imposed by the gob as well. Figure 6 depicts gob 
loading on the caving shield. In short, the shield is 



designed to provide resistance to (1) roof-to-floor displace- 
ments imposed by the weight and convergence of the over- 
burden strata, (2) face-to-waste horizontal displacements 
caused by displacement of the strata in the face area 
toward the gob, and (3) waste-to-face loading (displace- 
ments) imposed by gob loading on the caving shield and 
internal forces developed from horizontal components of 
the leg reactions. A two-dimensional representation of 
these three load conditions is illustrated in figure 7. The 
shield must also be designed with sufficient out-of-plane 
(in reference to the two-dimensional diagram depicted in 



Canopy 




Caving shield 



PLAN 



G3 







II 




ll 
ll 
ll 




!• 




ii 


i 


H 




ll 




H 








II 


II 


H 




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II 


ii 




II 

II 


II 
II 


H 
ii 








II 
II 
II 
II 
II 




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PLAN 



SIDE ELEVATION 




ii 
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S) 



SIDE ELEVATION 



Lemniscate links 







1 ! ! 


1 ! 1 


Centerline — *- 




i : : 


! ! 1 







Base 



) ii ii(p)i; 

I . . ii .1 i i I. 



PLAN 





PLAN 



Base 




SIDE ELEVATION SIDE ELEVATION 

Figure 4. -Shield component construction. 



figure 2) stiffness to resist lateral loading parallel to the 
face. In level seams, strata loading parallel to the face is 
unlikely, but lateral loading can be caused by uneven roof 
conditions along the face line, in which the canopy is not 
set parallel to the base (fig. 8). Pitching seams with face 
advance along a strike are more likely than nonpitching 
seams to have a lateral load component acting on the 
shields. 

The support contacts the roof and floor strata via the 
canopy and base, respectively. Loads are transferred 
between these components through either the leg cylinders 
or the caving shield-link assembly. Resistance to vertical 
loading is provided primarily (95 pet) by the leg cylinders, 



while resistance to horizontal loading is provided by both 
the leg cylinders and the caving shield-link assembly. The 
distribution of horizontal load between the leg cylinders 
and caving shield-link assembly depends on the degree of 
translational freedom in the numerous pin joints of the 
structure. As the support ages and these joints wear, the 
effectiveness of the caving shield-link assembly to resist 
horizontal loading decreases. 

Conditions that produce maximum loading of each sup- 
port component are summarized in figure 9. Overall, the 
worst condition for two-leg shield supports is standing on 
the toe of the base. This produces maximum stresses in 
the base structure, lemniscate links, and caving shield. 



Vertical shield loading 




Horizontal 

shield 

loading 



Vertical shield loading 
Figure 5.-Vertical and horizontal loads acting on shield support 



Maximum stresses in the canopy are developed when tip 
loading is a maximum, which occurs when the canopy is set 
with the tip in the air or under two-point contact at the 
ends of the canopy. Evidence of high tip loading can be 
high pressure in the canopy capsule or rotation of the 



canopy tip about the leg connection toward the floor. Leg 
pressure is an indication of overall shield loading and, 
obviously, leg loading. Leg pressure increases with vertical 
displacements and face-to-waste horizontal displacements. 



Gob loading on 
caving shield 




Figure 6. -Gob loading on caving shield. 



LU-LfJJ^ 



Vertical 
dispacement 



Increasing 
load 



No load 





Conopy set parallel 
to base 



Canopy not set parallel to base 



Figure 8. -Lateral loading caused by uneven roof conditions 
along face. 



CONTACT 
CONFIGURATION 



Face -to waste Increasing Increasing 
horizontal displacement load load 




Waste-to- face \ Increasing 

horizontal displacement \ load 

Decreasing 
load 



Figure 7. -Load conditions (displacements) imposed on 
shield support. 



EXPECTED 
SUPPORT RESPONSE 

Full canopy and base contact. 
Use as control standard. 



Maximum canopy stress due to 
bending. 



Potential large stresses in base 
due to bending with increased 
link loading. 

Worst case configuration for 
two- leg shield. Maximum stresses 
in base and lemniscate links. 

High link stresses as links must 
maintain stability of base 
structure. 



Maximum shear stresses in leg 
socket base connection 



Figure 9. -Conditions that produce maximum loading in support 
components. 



FAILURE MECHANISMS 



"Failure" is used in this discussion in the context of of the miner. Failure does not necessarily imply complete 

support performance that differs from design intentions. loss of support capability. Three types of support failures 

Failure causes diminished performance, which potentially are discussed: (1) inadequate capacity, (2) structural 

renders the support ineffective or hazardous to the safety failure, and (3) instability. 



INADEQUATE SUPPORT CAPACITY 

When support resistance is insufficient to prevent 
convergence of the strata beyond the hydraulic yield capa- 
bility of the leg cylinders, supports are said to "go solid," as 
illustrated in figure 10. In this configuration, the supports 
usually cannot be advanced and often are abandoned in 
place. With the trend toward higher capacity supports, 
abandonment of support systems because of inadequate 
capacity is now a relatively rare occurrence. 

Assessment of support capacity can best be made by 
observing leg pressures. Obviously, if leg pressures never 
reach yield pressure, support capacity is more than 
adequate. Loading of supports to yield load should not 
structurally damage the support, and occasional yielding of 
legs during a support cycle should not be judged as an 
indication of inadequate support capacity. As long as the 
support maintains effective ground control, occasional 
yielding is acceptable. 

Concern should be raised when yielding becomes persis- 
tent and more frequent as the mining cycle progresses. If 
this occurs on isolated supports, it is likely that there is 
simply a mechanical problem with those particular sup- 
ports. The greater concern occurs when the majority of 
the face supports yield excessively. It is this situation that 
indicates inadequate support capacity. 



Support (hydraulic) yielding produces a sound that can 
be heard by a person standing near the support. The 
sound is produced by hydraulic fluid passing through the 
yield valve. There may also be some mechanical noise 
associated with the change in geometry of the structure. 
Yielding is a means for the support to relieve load. In 
normal operation, the support yields slightly for a second 
or two and then recovers and continues to provide resis- 
tance to strata loading. Supports that fail to recover 
quickly probably have defective yield valves. 

STRUCTURAL FAILURE 

Structural failure can be attributed to either fatigue or 
component loading beyond its design strength. Fatigue 
occurs from repeated loading. Under the action of cyclic 
loads, cracks can be initiated as a result of cyclic plastic 
deformation. Even if the nominal stresses are well below 
the elastic limit, locally the stresses may be above yield 
because of stress concentrations at inclusions or mechani- 
cal notches. Consequently, plastic deformation occurs 
locally on a microscale, but it is insufficient to be described 
in engineering terms using strength of materials concepts 
(5). Mechanically, fatigue causes crack formation and 
promotes crack growth as the number of loading cycles 
increases. When a crack grows large enough (reaches 







Figure 10. -Shield supports going solid from inadequate capacity. 



10 



critical crack length), crack growth becomes unstable and 
failure (fracture) occurs. Fatigue failures generally form 
in weldments because of stress concentrations associated 
with material discontinuities or inherent flaws in pour 
welds. Fatigue failures develop gradually, but a fracture 
that results in loss of load-carrying capability can be 
sudden, being caused by one more application of load. 
Therefore, fatigue failures can be difficult to detect and 
can be potentially catastrophic. 

Structural failures from loading beyond design strength 
also develop gradually, but they are easier to detect than 
fatigue failures. The primary failure mechanism is general 
yielding, in which cumulative deformations are sufficiently 
widespread to threaten the structural integrity or designed 
function of a component. Failure (fracture) by static 
loading is unlikely since longwall roof supports are gen- 
erally constructed of mild steel, which exhibits good duc- 
tility. This means that the member will plastically deform 
or develop plastic hinges to effectively transfer the load 
before reaching ultimate strength and rupturing. For 
example, canopy or base sections may permanently deform 
(bend) an inch or more during plastic deformation. The 
general yielding observed in these members may not des- 
ignate areas of maximum stress. Maximum stresses gener- 
ally develop around holes, such as pin clevises, or in areas 
where the geometry of structure changes drastically. 
Deformation in these areas is usually difficult to detect 
since it is confined to a small area. These localized stress 
concentrations may not affect the overall structural integ- 
rity of the component. Conditions that induce maximum 
loading in support components were identified in figure 9. 

Base Failures 

Base failures seem to be the most prevalent type of 
failure and usually occur from fatigue after the support has 
been in operation for several panels of extraction. A 
common failure mechanism is for the leg socket casting to 
break away from the base structure. Formation of this 
failure is difficult to detect while the support is in service, 
as the leg socket is housed deep inside the base structure 
and this area usually is full of debris. Once the leg socket 
breaks loose, the support quickly becomes inoperable. 
The bottom plates of the base have insufficient strength to 
withstand the leg forces, and the leg cylinder rips the base 
apart by tearing off the bottom plate. 

Failure of the base structure (plates) can also occur 
without failure of the leg sockets. The probable failure 
mechanism is bending of the base. This is more likely to 
occur in minesites that have very strong immediate floor 
strata. In these hard floor conditions, the shearer may 
leave steps in the floor, as it is difficult to maintain a 
constant height of extraction from cut to cut. The base 
structure is then simply supported in two locations and is 



flexed as loading is applied. Repeated flexure causes the 
base to deform (plastically) or promotes fracture from 
fatigue, which eventually results in failure of the base 
structure. In softer floor conditions, the strata deform to 
provide a fuller contact to the base, which alleviates much 
of the bending and reduces the risk of failure. 

Standing the support on the toe of the base can also 
result in damage of the base structure. This configuration 
causes maximum stresses in the toe region, and the base 
deforms (bends) usually where the cross section is a 
minimum in the section of the base forward of the leg 
connection. 

Internally, the base structure is constructed with 
stiffeners that hold the top and bottom plates apart to 
form a beam arrangement that gives the base its bending 
and shear strength. Cases have been reported in which 
these stiffeners were not properly welded in place or the 
dimension tolerances were not within specifications. In 
these cases, the stiffeners broke loose and the base 
structures collapsed. This problem appears to be largely 
a matter of quality control, but it is critical to support 
safety. Since the stiffeners are hidden inside the base 
structure, it is virtually impossible (excluding X-ray or 
ultrasonic inspection) to see these deficiencies prior to 
failure. 

Canopy Failures 

Canopy structures are constructed of stiffened top and 
bottom plates similar to those of base structures, and 
hence, they are susceptible to bending-induced failures as 
well. Structurally, canopies are less stiff than bases, 
making them more susceptible to failure from bending 
than base structures. However, while permanent deforma- 
tion of the canopy is a fairly common occurrence, destruc- 
tion of canopies appears to be less frequent than observed 
destruction of bases. This suggests that canopies are less 
often subjected to critical bending. Three reasons why 
canopies might avoid critical loading are (1) immediate 
roof strata are usually partially fractured, and full contact 
with the canopy is more easily obtained, which minimizes 
bending moment; (2) tip loading on the canopy is usually 
smaller than toe loading on the base, since the resultant 
force is more likely to be located near the toe of the base 
than near the tip of the canopy, and (3) the canopy surface 
area is larger than the base area, allowing the canopy to 
distribute load more efficiently. 

Another common deformation of the canopy is 
"wrinkling" of the top plate between the internal stiffeners. 
This is probably due to concentrated loading at locations 
between the stiffeners but might also be an indication of 
failure of the weld that holds the stiffeners in place. If 
the stiffeners are not secure, the plate may buckle from 
forces developed within the plane of the plate thickness. 



11 



Failures of Caving Shield And Links 

Link members have become considerably more robust 
in shield designs of the past 10 years, and failures have 
been substantially reduced. Since the caving shield-link 
assembly has very little vertical load capacity (stiffness), 
links are not highly stressed for most load conditions. 
Almost all link failures can be attributed to conditions or 
operating practices that promote standing the support on 
the toe of the base or conditions that cause large horizon- 
tal displacement of the canopy relative to the base. 

Failure of the structure is most likely to occur in the 
region near the pin hole located on each end of the mem- 
ber. The failure mechanism is most likely crack formation 
somewhere on the circumference of the hole from local- 
ized high stress development. The pin holes elongate from 
continued wear and contact with the higher strength link 
pins. This results in point loading of the pins and high 
stress development at the contact areas. These failures are 
difficult to detect since this area is obscured from view by 
the caving shield clevis. Although link failures are rare, 
they can be catastrophic, since the links provide horizontal 
stability to the support structure. 

Likewise, caving shield failures are fairly rare but are 
more likely to occur than link failures. While links are 
designed primarily for axial loading only, shield mechanics 
indicate the primary loading mechanisms for the caving 
shield are bending and torsion. Maximum stresses and 
failure are most likely to occur in the clevis areas, where 
pins connect the link members to the caving shield. Some 
general yielding by bending deformation of the caving 
shield structure may also occur. 

Leg Cylinder Failures 

Assuming the face area is sufficiently stable to prevent 
violent outbursts of energy (bumps), it is unlikely that leg 
cylinders will experience structural failure since they are 
designed to control loading by hydraulically yielding at 
specified pressures. The most common failure associated 
with leg cylinders is seal leakage. Evidence of seal leakage 
is a gradual drop in pressure after the support is set 
against the roof. 

Another potential failure mechanism for hydraulic leg 
cylinders is for the yield valves to malfunction, allowing 
leg pressure to increase beyond design levels. Usually, the 
excessive pressure causes seal leakage, so that it is unlikely 
that sufficient pressure would develop to rupture the cylin- 
der casing. 



Supports utilized in bump-prone areas should use high- 
volume yield valves to allow quick discharge of fluid in 
order to prevent excessive development of pressure in the 
leg cylinders. Dynamic loading of supports without the 
proper yield valves can result in structural damage to the 
leg cylinders. 

INSTABILITY 

The shield is designed to provide stability against 
vertical, horizontal, and lateral loading. Because of the 
geometry of the structure, its weakest capability is against 
lateral (parallel to the face) loading. However, lateral 
loading is likely to be the smallest component of loading 
as the strata are least inclined to displace in a lateral direc- 
tion. Likewise, shield mechanics do not promote internal 
force developments in the lateral direction. In addition, 
lateral stability is enhanced by adjacent shields. 

Assuming there is no structural damage, it is unlikely 
for state-of-the-art shields to become unstable for vertical, 
horizontal, or lateral loading regardless of the canopy and 
base contact configuration. Some configurations produce 
temporary instability that causes the canopy or base to 
move slightly, but this movement is sufficient to adjust the 
contact configuration to a more stable configuration. 
Standing the support on the toe of the base, particularly 
with gob loading on the caving shield, is the most unstable 
configuration for two-leg shield supports. 

Obviously, when there is structural damage of any kind, 
the internal stability of the support structure is com- 
promised. Situations such as those previously described, 
where components are destroyed or their structural 
integrity is threatened, can easily render the support 
unstable. In addition to the base failures previously 
described, another failure mechanism that leads to dan- 
gerous instability of the shield is failure of the link pins. 
The link pins can be subjected to high shear forces from 
twisting of the caving shield promoted by unsymmetric 
loading or by large reactions developed from horizontal 
displacements of the canopy relative to the base. If the 
link pins in either the front or rear links fail, the shield 
would collapse under its own weight (geometric instability). 

Failure of the leg cylinders would also constitute a 
stability problem, but as indicated, leg cylinders are 
unlikely to fail from structural damage. Leg cylinders are 
also protected by internal check valves that prevent 
pressure loss in the event of rupture of the hydraulic feed 
line. Designed yielding of the leg cylinder does not 
promote shield instability. 



12 



SAFETY PRECAUTIONS 



This information circular attempts to provide generic 
evaluations of support safety. However, assessment of 
support failures is site specific and the magnitude of the 
problem must be considered in making judgments about 
support safety. The information provided here can help 
identify problems and suggest solutions, but the final 
judgment must be made with considerations of the severity 
of the problem, number of supports affected, past perfor- 
mance history, face conditions at the time of failure, and 
overall situation at the minesite. Finally, the best policy 
is always to correct problems as soon as they occur. How- 
ever, since this is often impractical and at times impos- 
sible, these generic recommendations regarding safety 
precautions for problem support systems are made with 
the realization that they are not applicable for all 
circumstances. 

SUPPORT CAPACITY CONSIDERATIONS 

When excessive yielding occurs on most of the face sup- 
ports because of inadequate capacity, some steps should be 
taken to reduce support loading. Operationally, there are 
several courses of action. First, it is generally a good 
practice to accelerate face advance (mining rate) as much 
as possible during times of excessive support loading. To 
some extent, subsidence of the overburden is time 
dependent, and accelerated advance rates may reduce 
loading caused by time-dependent failure of intermediate 
strata. Second, strata behavior and subsequent support 
loading are dependent upon support-setting loads. Gen- 
erally, there is an inverse relationship between strata 
convergence and support-setting forces, as shown in fig- 
ure 11, where face convergence decreases with increased 
setting forces. Since support loading after being set 
increases with increased convergence in proportion to the 
stiffness of the support structure, reducing convergence by 
increasing the setting force (leg pressure) reduces overall 
support loading. However, the total support load is the 
sum of setting load and subsequent load due to conver- 
gence. Therefore, increasing the setting load is effective in 
reducing overall support load only if the reduction in load 
subsequent to support setting is greater at the higher 
setting force than the increase in setting load. Conversely, 
it may be possible to reduce total support load by 
reducing the setting force if strata convergence is small 
and setting forces are high. In most circumstances, exces- 
sive support yielding occurs when support resistance is 
inadequate to effectively control the strata, and an increase 
in setting pressure probably is the most effective means of 
control, but the relationship between total load and setting 
force should be understood when making judgments of 
optimum setting forces. 



Questions are often raised concerning situations where 
one leg of a support is inactive or operates with diminished 
resistance. Obviously, this reduces the capacity of the 
support, but usually the support will remain functional and 
stable. From a load distribution viewpoint, loads will 
transfer down the side of the structure with the active leg. 
It is unlikely the leg pressure in the remaining leg will dou- 
ble, since additional loading imposed by the strata will be 
shared by adjacent supports. No large increase in com- 
ponent stress development (normalized to leg pressure) is 
likely, although there may be some increase in component 
stresses due to lateral loading if full canopy contact cannot 
be achieved at setting, producing unsymmetric load devel- 
opment. Stress development in this configuration should 
not exceed the design strength of the structure or pose any 
danger to the structural integrity of the support. 

Improper leg behavior due to seal leakage or malfunc- 
tioning control systems, when isolated to one or a few 
supports and when at least one leg (in two-leg shields) is 
functioning properly, should be corrected but usually does 
not pose an immediate safety danger. Generally, repairs 
can be delayed until scheduled maintenance periods. 
Improper leg behavior in isolated supports, in which all 
legs are not performing to design standards but provide at 
least 50 pet of design capacity, also usually does not pose 
an immediate safety hazard, but the problem should be 
corrected at the earliest convenient time. Finally, dimin- 
ished leg behavior on the majority of the face supports that 
results in less effective ground control should be corrected 
immediately. 



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SETTING LOAD (P s ) 



Figure 11. -Relationship between strata convergence and 
support setting force. 



13 



STRUCTURAL FAILURE AND STABILITY 
CONSIDERATIONS 

As previously indicated, components on most supports 
are constructed of mild steel (40 to 60 ksi yield stress) that 
is capable of large plastic deformation without fracturing. 
Hence, canopies and bases that are permanently deformed 
but do not show any signs of fracture along welds or 
elsewhere generally do not pose an immediate safety 
hazard. Yet, the significance of deformed members must 
be understood. While permanent deformation does not 
lower the yield strength of the material, members that are 
deformed by stressing beyond the elastic limit of the 
material (plastic deformation) maintain residual stresses 
after being unloaded. The residual stresses make the 
stress-strain curve more nonlinear so that the material will 
strain more for a given load increment, which means that 
it then takes less loading to further deform the member. 
Essentially, the member is weaker once it is deformed and 
gradually becomes weaker and weaker as the residual 
stresses build up from continued plastic deformation by 
loading beyond the material's yield stress. Therefore, 
while deformed members may not constitute an immediate 
safety hazard, they should be a warning of potential prob- 
lems that eventually may seriously threaten safety. 

Most higher strength steel constructions are used in 
canopies for support applications in low seams. These 
constructions use steels of 80 to 100 ksi yield stress, which 
exhibit much more brittle failure than the more commonly 
used mild steel constructions. Members constructed of 
these high-strength steels are much more susceptible to 
fracture from static loading beyond the elastic range. 
Therefore, deformation and crack formation are much 
more critical to support safety for components constructed 
of high-strength steels. 

Crack formation in any part of the support structure is 
a sign of potentially imminent danger. The crack indicates 
that the steel has failed. Whether this crack will propagate 
to cause destruction of a support component depends on 
many things, most notably, the ability of the member to 
effectively redistribute loading within itself. This is what 
makes judgments of support safety in these situations 
difficult. In any event, cracks in a support structure should 
be closely monitored, and as soon as the structural 
integrity of the support component is threatened, the 
support should be taken out of service. 

Support resistance is provided primarily by the hydraulic 
legs. Hence, it is unlikely that the support will show any 
signs of diminished load-carrying capability (support 
resistance) while structural problems to other components 
are developing. The issue concerning these structural 
problems is more one of stability than of support resis- 
tance. An unstable support is more dangerous to the 



safety of the miners than one that goes solid from 
inadequate capacity. For example, failure of the link pins 
would do little to affect the capacity of the support, but as 
indicated previously, the support would collapse under its 
own weight from instability. 

HYPOTHETICAL FAILURES 

To help put these issues into perspective, some specific 
recommendations of safety precaution are described for 
hypothetical structural problems that are most likely to be 
encountered. Again, these are generic solutions to site- 
specific problems that require careful consideration. It 
should also be understood that shield supports have very 
little redundancy in their design; all components must 
function to provide a stable and safe support. While most 
components are paired (left- and right-side member), it is 
generally not wise because of this apparent redundancy to 
permit continued operation when only one component has 
failed. Finally, it is better to try to determine the cause of 
failure before supports are repaired and put back in 
service. Fixing one problem may create another by 
transferring loads to other parts of the structure. 

1. Low leg pressure in one leg cylinder .-Assuming the 
support continues to function and provide effective strata 
control, the operator can continue mining while investi- 
gating the cause of diminished leg resistance. If the 
problem can be corrected on routine maintenance shift, it 
should be. If it cannot be fixed on routine maintenance 
(for example, if the problem is determined to be seal 
leakage), the operator should continue mining until a 
convenient opportunity presents itself to change out the 
leg. If strata control does not diminish, operation can 
continue to panel completion, but leg reactions and 
support behavior should be closely monitored. 

2. Wrinkling of canopy top plate.-The operator should 
monitor periodically for crack formation in the plate 
structure and continue to operate until crack formation 
appears. After crack formation begins, the operator 
should change out canopies at earliest convenience; panel 
completion is usually acceptable. 

3. Bent canopy— The operator should continue to 
operate while monitoring for crack formation. The 
operator should inspect weldments and look for signs of 
failure where the tapered section of canopy begins and 
look for any indication that internal stiffeners are broken 
loose, such as large deformations running down the length 
of the canopy. If structural integrity of canopy seems 
threatened (that is, crack development and progression), 
the canopies should be changed out as soon as possible. 



14 



4. Leg socket broken loose from base structure. - 
Operations should be stopped and the base unit replaced 
immediately. This is a very serious problem. 

5. Significant crack growth in top or bottom plate 
weldments of base structure. -The, mine operator should 
replace the base units with stiffer design at first scheduled 
maintenance period. Obviously, if failure has resulted in 



destruction of the base unit, it must be replaced 
immediately. 

6. Sheared link pin -Operations should be stopped 
immediately and the pins replaced. The operator should 
examine for damage of adjoining clevises and repair or 
reinforce clevises as necessary. 



CASE STUDIES OF SUPPORT FAILURES 



A few case studies of support failure are described in an 
attempt to put the issues of longwall support safety into 
perspective. The names of minesites and support manu- 
facturers are withheld so as not to disclose any confiden- 
tial information. 

1. Leg cylinder casing failure. -Figure 12 shows one of 
the rare occurrences of leg casing failure. A section of the 




casing ruptured and blew out while the leg cylinder was 
under high pressure. Luckily, no one was injured. The 
cause of failure was attributed to damage done by welding 
of the hose inlet lug on the cylinder casing. 

2. Unstable shield design. -An entire face of four-leg 
shields was lost in 1988 because of stability problems. 



Split 
caving shield 




Figure 13.— Split caving shield design that failed from instability. 




Figure 12. -Failure of leg cylinder casing. 



Figure 14. -Permanent deformation of base structure. 



15 



These shields featured a split caving shield design as 
illustrated in figure 13. No apparent structural damage 
was observed prior to failure. The split caving shield 
reduces out-of-plane stiffness of the structure, which 
reduces the support's resistance to lateral (parallel to the 
face) loading. This design, coupled with wear in the pin 
joints, was the probable cause of failure. The entire face 
was abandoned in place. 

3. Bent base structure -An example of general yielding 
of a component by loading beyond its elastic design 
strength is shown in figure 14, where the toe region of a 
base structure is permanently deformed. Notice the rela- 
tively smaller cross section of the base, which provided 
inadequate bending strength. Standing the support on the 
toe of the base was identified as the load condition that 
produced the failure. The failure eventually rendered the 
support inoperable, and the base units had to be replaced. 

4. Leg socket failure. -An example of leg socket failure 
in the base structure of a two-leg support is shown in 
figure 15. The figure shows dye penetrant application to 
highlight the failure, which occurred in the weldments. 



These bases were removed and replaced in the middle of 
panel extraction, causing several weeks of downtime. 

5. Base stiffener failure. -Figure 16 shows a modification 
that was made to stiffen a base structure in the region 
between the front and rear link pin and to provide out-of- 
plane stiffness to the base structure. An interesting point 
concerning this failure was that the first sign of failure 
noticed underground was in the caving shield clevis area. 
The weakness in the base unit caused the base to bend, 
which in turn caused excessive link loading. This is an 
indication of how failure of one component can affect the 
loading of another component. 

6. Structural failure of canopy. -Figure 17 shows failure 
(crack formation) of a canopy structure near the canopy 
hinge, from loading beyond its design strength. Also 
shown in the figure is a finite element analysis of stress 
concentrations in this section. This is a good example of 
how development of localized high stresses in areas around 
holes causes failure. The hinge area was strengthened by 
adding a reinforcement plate in front of it. 




Figure 15.-Example of leg socket failure. 



16 




Area of modification 



Figure 16. -Base failure and subsequent modification. 



SIDE VIEW 



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Not to scale 



Figure 17. -Failure of canopy structure from high stresses in canopy hinge. 



17 



FUTURE RESEARCH EFFORTS 



The Bureau proposes to conduct controlled destructive 
testing of various shield supports in the mine roof 
simulator. The primary goal will be to assess the impact 
of failure on the safety of the support system. More 
specifically, the objectives of this research will be to 
identify conditions that cause failure and the effect of the 
failure on the load transfer mechanics of the support. 
Once this is understood, better judgments can be made as 
to the safety of support systems that have developed 
structural and mechanical problems. 



In addition, the Bureau is continuing current nonde- 
structive testing efforts to further enhance the knowledge 
of support mechanics. The intent of this research is to 
improve support selection and design and to further 
improvements in performance testing techniques. These 
efforts will minimize the risk of support failure with 
current supports and provide engineering guidelines to 
eliminate current failure mechanisms in future support 
designs. 



CONCLUSIONS 



Longwall mining has improved the safety and increased 
the productivity of underground coal mining. Ground 
control is an important consideration in longwall mining. 
State-of-the-art powered roof supports usually provide 
effective ground control, but failures of roof support 
structures still occur. These failures lower productivity and 
endanger the safety of the miners. 

This information circular provides insight into the 
causes of these failures and their impact on the capability 



of the support to continue to provide safety to the miners. 
The Bureau's research as presented in this report should 
provide assistance to MSHA and industry personnel in 
conducting safety evaluations of longwall roof support 
systems and should make mine managers and engineers 
more aware of the problems and consequences of support 
failures. 



REFERENCES 



1. Barczak, T. M., and D. E. Schwemmer. Horizontal and Vertical 
Load Transferring Mechanisms in Longwall Roof Supports. BuMines 
RI 9188, 1988, 24 pp. 

2. . Two-Leg Longwall Shield Mechanics. BuMines 

RI 9220, 1989, 34 pp. 

3. Barczak, T. M., D. E. Schwemmer, and C. L. Tasillo. Practical 
Considerations in Longwall Face and Gate Road Support Selection and 
Utilization. BuMines IC 9217, 1989, 22 pp. 



4. Barczak, T. M. Research on Shield Supports Using the Bureau's 
Mine Roof Simulator. Paper in MinTech '89. Sterling Publ., 
Mar. 1989. 

5. Brock, D. Elementary Engineering Fracture Mechanics. 
Martinns Nijhoff, 1986, p. 57. 



* U.S. GOVERNMENT PRINTING OFFICE: 611-012/00,076 



INT.-BU.OF MINES,PGH.,PA. 28916 



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